1. Field
The present embodiment relates to an ophthalmic surgical device for the removal of vitreous and other tissue from a patient's eye and, more particularly, to an ophthalmic surgical device that vibrates a cannula with at least one port to disrupt and aspirate vitreous and other tissue from the eye.
2. Description of the Related Art
Vitrectomy cutters (or simply vit cutters) are ophthalmic medical device accessories indicated for use in removing the vitreous humour (often referred to within ophthalmology as “vitreous” or “vit”) from the posterior segment of the eye, which lies between the lens and the retina. Sometimes vitreous is removed because it is contaminated with materials that degrade vision (e.g., blood from ruptured vessels, or other cell material, referred to as vitreous floaters, that create spots in the visual field). Other times vitreous is removed to provide surgical access to structures on or near the retina. Also, vitreous is removed to relieve tension exerted on the retina and other structures of the eye.
Vitreous is about 98% to 99% water, but it is bound together with vitrosin. Vitrosin is a “network of collagen type II fibers with the glycosaminoglycan hyaluronic acid” (taken from http://en.wikipedia.org/wiki/Vitreous_humour). Vitreous has a soft jelly consistency and a viscosity two to four times higher than water. The vitreous fibers or strands are anchored to the vitreous membrane (or hyaloid membrane) which rests, in part, next to the retina—pulling on the vitreous membrane can cause optical distortions or even damage to the retina as the vitreous membrane pulls away from the retina. From a fluid dynamic perspective, vitreous may be treated as being thixotropic, exhibiting shear thinning, and as a ground substance, because it is a water based substance containing glycosaminoglycans. Thus, vitreous is an extracellular material in the body classified as thixotropic (see Wikipedia.org for Ground Substance and Thixotropy).
The long collagen fibers create a gelatinous consistency and prevent the vitreous from being aspirated out of the posterior section directly (without prior disruption), in at least three ways. First, the vitreous fibers pull along enough material to prevent vitreous from being drawn into a small hole directly via vacuum aspiration. That is, a small portion will get drawn into the hole, pulling along a larger portion which will not fit through the hole, thereby clogging it. Second, even if, by using a large enough hole and a strong enough vacuum, some vitreous was successfully pulled into the aspirating device, the sticky nature of the vitreous would grab an inner wall of the aspirating device, reducing flow rates below surgically desirable levels. Third, even if a continuous flow were established for a short period of time, the ends of the vitreous strands not yet pulled through the hole will continue to pull material toward them, eventually pulling on and damaging other structures such as the hyaloid membrane or the retina; this is colloquially referred to as “traction” by retinal surgeons. A surgeon will not attempt to passively aspirate the vitreous directly without some form of dissection or disruption if they feel the risk of injury is high enough. For instance, when there is a dropped lens fragment into the posterior segment during surgery, it is common that a vitrectomy (removal of the vitreous) will be done before the lens fragment is removed via phacoemulsification; this is to eliminate the dangers of traction that could occur if phacoemulsification were attempted in vitreous.
Various amounts of vitreous may be removed depending on the disease state being treated. Vitreous is typically removed from the center of the globe to provide access to various areas around the posterior surface of the eye. Vitreous is removed from areas that the surgeon needs to access for therapeutic reasons—for instance, to provide safe direct access to membranes that cover and obscure specific retinal regions. Vitreous is also removed from areas the surgeon identifies as necessary for prevention of future damage to the retina from traction or pulling. In these last two instances, the surgeon will want to remove as much vitreous as possible from specific areas that may be close to the retina.
In most instances, access to the vitreous is gained through the sclera. In some instances, referred to as vitreous prolapse, the posterior wall of the capsular bag holding the lens is ruptured during cataract surgery using ultrasonic phacoemulsification (phaco). In these cases, vitreous in the anterior segment and some vitreous from the anterior portion of the posterior segment may be removed through a corneal entry. It is current clinical practice that a separate vitrectomy device must be used to remove vitreous, instead of the ultrasonic phacoemulsification (phaco) device. If the surgeon attempts to remove the vitreous with either the ultrasonic device using the lens removal tip or an irrigation/aspiration handpiece using the capsule polishing tip, the handpiece needles become clogged by the sticky vitreous and generate traction on the elements in the posterior section of the eye (for the reasons noted above) and become ineffective. It is generally acknowledged in the industry that vitreous cannot be removed from the anterior chamber using a phaco device with a standard tip.
Many patents relating to ultrasound describe breaking ocular tissue in general and lens tissue specifically into fragments or pieces. When considering removal of the lens, describing it as slurry of broken lens fragments mixed with the irrigation fluid provides a fairly accurate model. Given the stringy, sticky, gelatinous nature of the vitreous, this is a less accurate description of the tissue.
In light of the above, a primary design objective of devices for vitreous removal is to break up the vitreous strands, permitting aspiration into a cutter, improving flow through the cutter, and minimizing traction outside the cutter. An additional objective is to minimize the distance between the aspiration port and the end of the device, so that, as long as the low traction target is achieved, vitreous can be removed from regions as close to the retina as possible.
Clinically, the user wishes to achieve five objectives: Remove the vitreous quickly, enter the eye through as small a wound as possible, avoid mechanical damage to the retina from traction or direct cutting, minimize the infusion pressure in the eye, and maintain a stable and positive pressure in the eye. Slow removal of vitreous means longer surgical times, which are stressful for the patient and the patient's eye. Large wounds require stitches across the wound for closure, potentially causing discomfort and optical distortion. Mechanical retinal damage may result in blind spots or chronic vision degradation. High infusion pressures may restrict blood flow to the retina, potentially causing permanent damage to the retina. Fluctuations in intraocular pressure may cause tissue to move into the mouth of the cutter inadvertently, or cause the eye to collapse momentarily. Furthermore, it is possible for bubbles to form at the tip of an ultrasonic cannula when brought into contact with vitreous thereby obscuring vision of the surgical site and adversely affecting the fluidics within the eye. These bubbles, commonly referred to as cavitation, also may damage tissue not intended to be damaged.
These objectives may conflict with each other. In general, tissue aspiration paths must get bigger to speed up vitreous removal; larger aspiration paths, in turn, require larger wounds to insert a cannula, and require higher infusion pressures to support water flow into the eye to keep the intraocular pressure stable. Low infusion pressures provide less safety margin for intraocular pressure fluctuation. A further complicating factor is that vitreous flow for a given pressure differential is generally lower than water flow; and vitreous and water are hard to distinguish visually during surgery, as they are both transparent. The infusion pressure must be set high enough to keep the chamber stable if a tissue cutter's mouth gets into water, or the aspiration vacuum must be set at a low level, minimizing vitreous flow and risking clogging of the tissue cutter. Therefore, it would be desirable to provide a surgical device that allows use of an infusion pressure near normal physiological intraocular pressure levels and still achieve satisfactory vitreous flow through a small lumen while maintaining stable pressure in the eye during surgery.
There have been patents and scientific articles that mention removing vitreous with an ultrasonic device but none have taught how to safely and reliably remove vitreous without traction during surgery.
U.S. Pat. No. 3,805,787 by Banko, discloses removing vitreous with an ultrasonic device. The device includes a shield to confine the ultrasonic energy and provide a safety factor by keeping tissue not to be removed away from the ultrasonic probe, such as protecting the retina. There is no discussion regarding traction of vitreous during removal.
U.S. Pat. No. 3,941,122 by Jones, teaches removing vitreous gels from a physically small, high frequency source, preferably pulsed. The frequency of operation is “on the order of at least 90-100 MHz”, considerably higher than conventional 20 to 60 kHz frequencies employed in standard ophthalmic microsurgical systems. Furthermore, the transducer is identified as being located in the radiating tip itself. There is no discussion regarding traction of vitreous during removal.
U.S. Pat. No. 4,531,934 by Kossovsky et al., teaches fragmenting and aspirating ocular tissue, including vitreous, using ultrasound and a needle with a single opening at one end with a diameter substantially less than the diameter of the axial bore of the needle. It includes a “transverse end wall portion . . . opening and bore . . . joined together . . . to create a vacuum to aspirate the ocular tissue”, or aspiration without assistance of an aspiration pump, which could result in unacceptably low flow rates. There is no discussion regarding traction of vitreous during removal.
U.S. Pat. No. 4,634,420 by Spinosa et al., relates primarily to an ultrasonic system with an improved removable sheath device for delivery of treatment fluid. Reference to use on vitreous is mentioned. There is no discussion regarding traction of vitreous during removal.
U.S. Pat. No. 6,126,629 by Perkins, discloses a phaco-emulsification needle with multiple ports, including an axial port, i.e. a port on the apex of the distal tip, which is safe near the posterior capsule so that vitreous prolapse does not occur. There is no discussion regarding traction of vitreous during removal.
U.S. Pat. No. 6,299,591 by Banko, describes a phacoemulsification instrument, including several embodiments of needles with different geometrical tips and aspiration ports. The different tip designs are for concentrating the ultrasonic energy as desired. There is no discussion regarding traction of vitreous during removal.
US 2007/0255196 by Wuchinich, describes an ultrasonically vibrated solid tip surrounded by a stationary sheath for liquefaction of vitreous. There is no discussion regarding traction of vitreous during removal.
Studies have been published on the use of ultrasound in vitreous, without simultaneous irrigation and aspiration. For instance, in Ultrasonic Vitrectomy—an Alternative Technique to Presently Used Mechanical Procedures (Lietgeb, Schuy, and Zirm in Graefes Archives of Clinical and Experimental Ophthalmology, volume 209, pages 263-268, 1979) the authors used a 2 mm diameter probe at 60 kHz with an unknown stroke located in the middle of the posterior chamber to liquefy bovine vitreous, and measured the diameter of the liquefied regions around the probe's distal tip. However, no attempt was made to aspirate the vitreous out of the chamber through the device. There is no discussion of traction of vitreous during removal.
Mechanical vit cutters having an inner cutter that is movable relative to an outer cutter are well known and are essentially the only type of vit cutter used. Virtually all mechanical vit cutters are of the guillotine-type with an axially reciprocating inner cutter. There are however, examples in the prior art of inner cutters that rotate or oscillate back-and-forth across a port on the outer cutter. The oscillating cutters are not used because of potential traction problems from uncut vitreous (“spooling”) that could cause damage to the retina. In all cases, mechanical vit cutters rely on aspiration to pull vitreous into the cutter port and a reliable scissors-type contact between the inner and outer cutters is required to prevent traction. Typically, pneumatic drives have been used to create the axial inner needle motion; electric drive designs have also been proposed or marketed using motor driven cams, voice coil, solenoids, or low frequency non-resonant piezoelectric actuators. David Wuchinich has proposed a version on his website where the inner needle is driven by a piezo-electric element in a resonant transducer. Despite the differences in drive mechanisms, all of these devices consist of a stationary outer needle with a port and a moving inner needle.
Recently, the frequency of the cutting action of mechanical vit cutters has been increased and the period between cuts has decreased to reduce the overall size of the pieces of cut strands. Cut rates have advanced from 600 CPM (100 msec per cut cycle) to 5,000 CPM (12 msec per cut cycle) and there are active efforts to increase the cut rate to 10,000 CPM (6 msec per cut cycle). The ultimate maximum cut rate will be limited at some point, by the reciprocating mass and by the volumes of air that must be moved back and forth in the pneumatic devices and the motor requirements in electrical devices.
Necessarily, all mechanical vit cutters with needle pair designs include two needles, an outer needle and an inner needle. The aspiration path is routed through the inner needle, and the geometry of the aspiration path is determined, in part, by the inner needle inner diameter (ID). Because the inner needle must move relatively freely inside the outer needle, the effective separation between the inner cutter OD and the aspiration path OD must be two tube wall thicknesses plus some air gap. Ophthalmic surgical instrumentation has been getting smaller, to permit use of smaller incisions, which leak less, heal faster, do not require sutures, require less preparation time, and induce fewer optical aberrations. However, because of this trend, there is user interest in making the OD of the outer cutter smaller. Since (within the basic model for flow in a tube) resistance is proportional to the fourth power of the tube diameter, the use of a second, smaller inner tube to provide the aspiration path limits the aspiration rate by increasing the flow resistance and decreasing the flow rate.
Because the mouth of the outer needle's port must be large enough for a reasonable amount of intact vitreous to be pulled in past the outer needle wall so that it can get trapped and cut by the inner needle and the outer port edge, some of the pieces of vitreous may have a cross sectional area about the same size as or larger than the inner diameter of the outer needle. Therefore, the cut pieces of vitreous are necessarily larger than the aspiration path defined by the ID of the inner needle. This means that the vitreous pieces will drag the inner needle walls and may, from time to time, jam together as they flow up the tube. This increases the flow resistance and the likelihood of clogs, while also decreasing the effective flow rate.
In order to cut effectively, the forward edge of the moving inner needle must extend past the forward edge of the port in the stationary outer needle, while staying pressed hard against it. Because of the desire for both complete cutting of the vitreous to minimize traction, and for the forward-most possible position of the port, designers and manufacturers find themselves balancing the likelihood of an occasional incomplete cut (because the needle end fails to pass the port end) against the inability to cut close to the retina (because the port is located further back from the distal end to provide more room for the inner needle to drive past the end of the port). All mechanical vit cutters rely on some level of interference between the inner needle and the outer needle due to bending or displacement of the inner needle; this interference adds drag, which slows down the inner needle, and makes higher cut rates harder to achieve.
High speed video of vitreous being cut by guillotine cutters has shown that, as the inner needle passes over the port and squeezes the vitreous against the leading outer port edge, the vit cutter pulls on the vitreous outside the port, moving it a distance equal to about the port mouth size, which is typically around 0.015″ (381 μm). This creates traction (pulling on the vitreous outside the port beyond the natural flow of vitreous to the port) during each cut, even during perfect cuts.
Flow measurements have shown that the flow rate of water through the current mechanical vit cutters is much higher than the flow rate of vitreous through the same cutters at the same vacuum levels and actuation rates. This indicates that the flow resistance of the vitreous is higher than the flow resistance of water, which has two effects. It makes the overall vitrectomy time longer, and it causes abrupt changes in irrigation flow into the eye as the cutter moves between water and vitreous, and back again. These abrupt flow changes require higher infusion pressures to manage the intraocular pressure, and potentially could cause damage to the structures in the eye.
As noted, surgeons would like the port to be located as close to the end of the cutter as possible, to facilitate removal of vitreous close to membranes that are close to the retina. However, in conventional mechanical vit cutters, the designer must leave space between the forward edge of the port and the end of the outer needle, so that the inner needle has room to pass by it, accounting for all assembly variances and tolerances. This means the forward edge of the cutter port may be located about 0.008″ to 0.015″ (200 to 380 μm) from the end of the outer needle.
Although partially effective, all the prior art vitreous removal devices fail to fully realize the end goals of small wound size, high flow, and low traction.
Corresponding reference numerals indicate corresponding parts throughout the several views of the drawings.